Question

I’m trying to get better understanding over the Face detection implementation by openCV.

I have seen this command over haar.cpp file:

stage_sum += classifier->alpha[sum >= t];

somehow , I can’t figure the meaning of this command, I know what += means , what does the alpha[sum >= t ] means ?

Deceleration are like this:

    double t ,sum ; 
    float stage_sum ; 
    CvHidHaarClassifier* classifier;

    typedef struct CvHidHaarClassifier
    {
        int count;
        //CvHaarFeature* orig_feature;
        CvHidHaarTreeNode* node;
        float* alpha;
    }

    typedef struct CvHidHaarFeature
    {
        struct
        {
            sumtype *p0, *p1, *p2, *p3;
            float weight;
        }
        rect[CV_HAAR_FEATURE_MAX];
    }
    CvHidHaarFeature;


    typedef struct CvHidHaarTreeNode
    {
        CvHidHaarFeature feature;
        float threshold;
        int left;
        int right;
    }
    CvHidHaarTreeNode;

Answer 1

The term

sum >= t

returns either 1 or 0, depending on whether the sum is greater/equal than t or not. This value (0 or 1) is used as an index into the array classifier->alpha[] (which has only two entries).

t is a threshold learned during training the (weak) classifier. During testing (i.e. classifying an image position whether it is a face or not), it is tested for each weak classifier, whether the value of sum is greater than the threshold t, and depending on that, a different value is added to stage_sum.